The present invention relates to a method and an apparatus for inspecting the surface of a substrate e.g. a semiconductor wafer etc. on the basis of a scattered light from dust or rubbish etc. on the substrate surface.
In the fabrication of semiconductor devices, one generally detects particles attached on the surface of the semiconductor wafer in order to improve yield of semiconductor chips obtained from a semiconductor wafer. Hitherto, such a detection is achieved by irradiating a laser beam etc. on the surface of a substrate e.g. a semiconductor wafer etc. to focus a scattered light from the substrate surface to detect the scattered light thus focused by using a photo-multiplier tube. For instance, where the substrate is a silicon wafer, since an output voltage based on a scattered light from particles on the substrate monotonously increases as indicated by the sensitivity characteristic curve A in FIG. 18, detection is made by setting a slice level to a level with respect to a particular particle diameter, thus to judge the particle diameter on the basis of the output voltage of the photomultiplier tube.
However, in the case where thin films of other compounds are formed on the surface of the silicon wafer, it is difficult to determine such slice levels. The sensitivity characteristic curve B in FIG. 18 shows the case where a polysilicon thin film is formed on the surface of the silicon wafer. Likewise, sensitivity characteristic curves C and D show the cases where thin films of a silicon oxide and a silicon nitride are formed thereon, respectively. Because the optical properties, particularly reflectivity or reflection factor characteristics of respective thin films are different from each other, each sensitivity characteristic curve differs from that of the silicon wafer, with the result that one becomes difficult to determine the slice level and it is unable to detect particles with the same sensitivity as that measured with the silicon wafer.